The spatial and temporal control of gene activation and silencing is essential for proper morphogenesis of developing organs. Recently, miRNA (microRNA)-mediated gene silencing has emerged as a major mechanism of gene silencing in several species. It has been estimated that at least 1% of the transcripts in the genome code for miRNAs, and that miRNA have at least 20-30% of the coding genes as their targets. Genetic inactivation of components of the miRNA-mediated gene silencing machinery, such as Dicer, Argonautes, or of specific miRNAs in multiple organisms support the idea that miRNA-mediated gene regulation is essential in the developing mouse. The role of miRNA-mediated gene silencing in complex branching organs, such as the lung, has not been explored. The objective of this application is to explore the role of miRNA-mediated gene silencing in lung development. The central hypothesis is that miRNAs can selectively turn expression of target genes off to allow normal morphogenesis and differentiation. To test this hypothesis we propose to: Identify and characterize expression of microRNAs in the developing lung (Aim 1), investigate silencing of specific miRNA targets and its consequences in lung cell cultures in vitro and in vivo (Aim 2), characterize the function of the RISC complex component Argonaute-1 in the developing lung (Aim 3). Relevance of the research to public health: Loss of control of normal gene expression has been associated with abnormal development and with many human diseases including cancer. Micro RNAs have been recently emerged as a major mechanism to control gene expression in normal development. Little is known how these molecules function in the embryonic lung and what genes they regulate. This study will provide insights into this issue and will help to understand mechanism of human diseases.